As the support for lithographic printing plate, there have been conventionally employed aluminum supports, but they have the following drawbacks, namely (1) they are expensive and (2) low in mechanical strength, and may sometimes be cut off when flexed at an acute angle in, for example, mounting on a cylinder of an offset printing machine.
On the other hand, as the support using a relatively less expensive steel material, there is disclosed in European Patent Publication No. 20021 an electrodeposited chromium layer on a metal plate characterized by an effective absence of generally planar exterior surfaces and relatively sharp protruberant angles. However, this support does not sufficiently adhere to the image portions and, when employed for a lithographic printing plate, the image line portion may partially be peeled off during printing over a long term, ink spreading may develop at the dots portion, scumming may develop at the non-image portions, or printing life is not sufficient. Also, because of the narrow tolerance in the developing method, there is practiced the so-called "hand processing" in which development is performed by rubbing the plate with a sponge impregnated with a developer without use of an automatic developing machine, whereby there is involved the problem that damages are liable to occur at the image portions. Further, separately from the treatment step for formation of an electrodeposited chromium layer, the step of treatment in a bifluoride grainer tank is required and it will take much time for this treatment step. As still other disadvantages, the total amount of electricity employed during formation of the electrodeposited chromium layer is very high which results in high production costs. Also the narrow tolerable temperature range in a bath for formation of electrodeposited chromium layer will necessarily require careful attention in temperature control.
Accordingly, an object of this invention is to provide a support for lithographic printing which is high in mechanical strength and also has sufficient performances in adhesion to a photosensitive layer, printing life and tolerance in developing methods. Other objects of this invention will become apparent from the following description.